Railroad tank cars transport bulk liquids such as oil or ethanol. Regulations require most railroad tank cars operating in North America be equipped with pressure relief devices. Pressure relief devices protect the tank from events such as product surges, over fills, and commodity reactions, but the primary purpose is preventing catastrophic tank failure by regulating pressure within the tank during a fire condition. The contents of the tank exist in a two-phase state—the liquid phase tank lading (commodity for transport) and the vapor phase above the lading. For normal ambient temperature liquid commodities the vapor phase is approximately atmospheric pressure. In a fire situation, heat input to the tank drastically increases the vapor pressure until it exceeds the pressure relief device safety setting and vapor is expelled.
Existing pressure relief devices are generally located near the center of the tank along its longitudinal center line. This location places the pressure relief device in communication with the vapor space above the tank lading when the tank is oriented in its normal operating position. A properly sized relief device is effective at relieving pressure during a fire situation by discharging vapor at a rate greater than or equal to the rate at which the pressure is generated.
In a derailment and roll-over situation, the tank car is no longer oriented in its normal operating position and the pressure relief device may no longer be in communication with the vapor space. Instead, the pressure relief device may be in communication with the liquid space. In this situation, the pressure relief device will expel liquid instead of vapor. Expelling liquid is problematic for several reasons. If the liquid is flammable, it may continue to fuel the fire and accelerate the temperature increase. Furthermore, because the pressure buildup is mainly caused by the vapor mass, expelling liquid does not relieve pressure at the same rate as expelling vapor. Additionally, a decrease in liquid mass increases heat transfer to the tank, further accelerating the pressure buildup.